U.S. patent number 4,817,605 [Application Number 07/027,715] was granted by the patent office on 1989-04-04 for pacemaker system and method for measuring and monitoring cardiac activity and for determining and maintaining capture.
This patent grant is currently assigned to Siemens-Pacesetter, Inc.. Invention is credited to Jason A. Sholder.
United States Patent |
4,817,605 |
Sholder |
April 4, 1989 |
Pacemaker system and method for measuring and monitoring cardiac
activity and for determining and maintaining capture
Abstract
A system for determining P-wave or R-wave capture in response to
pacemaker supplied electrical stimuli. One embodiment includes a
conventional bipolar atrial lead having a tip electrode, connected
to an atrial pulse generator circuit within an implantable
pacemaker, and a ring electrode, spaced apart from the tip
electrode, connected to a P-wave sensing EGM amplifier within the
pacemaker. The bandpass characteristics of the P-wave sensing EGM
amplifier allow detection of all electrical frequencies in the
atrium within the bandpass chosen. The output signal from this
amplifier is selectively telemetered to an external receiver, as is
a signal indicating the generation of an atrial stimulation pulse,
where the occurrence of atrial stimulation pulses and P-waves can
be monitored. In operation, if constant time intervals between the
monitored atrial stimulation pulses and P-wave occurrences are
present, P-wave capture has occurred. In the event of variable time
intervals, capture has not occurred and the magnitude of the atrial
stimulation pulses can be increased until P-wave capture does
occur. An alternative embodiment measures this time differential
and makes the stimulation pulse adjustment automatically. Other
embodiments use the same manual or automatic systems to determin
R-wave capture.
Inventors: |
Sholder; Jason A. (Canoga Park,
CA) |
Assignee: |
Siemens-Pacesetter, Inc.
(Sylmar, CA)
|
Family
ID: |
26702812 |
Appl.
No.: |
07/027,715 |
Filed: |
March 19, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
662723 |
Oct 19, 1984 |
4686988 |
|
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Current U.S.
Class: |
607/28 |
Current CPC
Class: |
A61N
1/368 (20130101); A61N 1/3712 (20130101); A61N
1/3714 (20130101) |
Current International
Class: |
A61N
1/368 (20060101); A61N 1/37 (20060101); A61N
1/362 (20060101); A61N 001/36 () |
Field of
Search: |
;128/419PG,419PT |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Gold; Bryant R. Miller; Leslie
S.
Parent Case Text
This is a divisional of co-pending application Ser. No. 662,723
filed on 10/19/84 now issued as U.S. Pat. No. 4,686,988.
Claims
What is claimed is:
1. A system for determining atrial capture comprising:
heart pacing means for generating stimulation pulses;
electrically conductive lead means for sending electrical
stimulation pulses to and receiving electrical signals from a
desired atrial location, said lead means including
a first electrically conductive electrode connected to said heart
pacing means, and
a second electrically conductive electrode spaced apart from said
first electrode;
an amplifier connected to receive signals sensed by said second
electrically conductive electrode, said amplifier including
receiving means for receiving signals indicative of any atrial
P-wave sensed by said second electrode, whether said atrial P-wave
is a naturally occurring P-wave or a P-wave resulting from the
application of a stimulation pulse applied to said first electrode,
said amplifier further having means for generating an output signal
indicative of the sensing of said atrial P-wave; and
means responsive to the presence of said output signal for
determining whether atrial capture has occurred, said means for
determining atrial capture comprising means for measuring
successive time intervals between application of an atrial
stimulation pulse through said first electrode and the generation
of the output signal by said amplifier, atrial capture being
indicative if said measured time interval remains constant and
corresponds approximately to a predetermined time interval.
2. The system recited in claim 1 wherein said first and second
electrodes are spaced apart at least one-half inch.
3. The system recited in claim 1 wherein the amplifier includes
means for amplifying frequencies within the range of 3 to 125
Hz.
4. The system recited in claim 1 wherein said lead means further
includes a third electrode electrically coupled to said heart
pacing means through which stimulating pulses can be provided for
stimulating the ventricle, and a second amplifier coupled to said
third electrode that senses R-wave signals originating in the
ventricle, said second amplifier generating a second output signal
indicative of the sensing of an R-wave at said second amplifier,
said second output signal being provided to said heart pacing means
to signal that an R-wave has occurred.
5. The system recited in claim 1 wherein there is further
included:
means for controlling the magnitude of the atrial stimulation
pulses of said heart pacing means including means for reducing the
magnitude of said atrial stimulation pulses until said atrial
capture-determining means indicates lack of capture, and then
increasing the magnitude of said stimulation pulses until said
atrial capture-determining means indicates capture.
6. The system of claim 1 wherein there is further included means
for controlling the magnitude of the atrill stimulation pulses
generated by said heart pacing means in response to said atrial
capture determining means.
7. A system for determining capture in response to pacemaker
supplied electrical stimuli provided to a heart comprising:
a lead having proximal and distal ends, said lead having a first
exposed electrically conductive electrode at said distal end, a
second exposed electrically conductive electrode near said distal
end, spaced apart and electrically isolated from said first
electrode, first and second terminals near the proximal end of said
of said lead, and first and second conductor means for electrically
connecting said first and second electrodes, respectively, to said
first and second terminals; and
pacemaker means comprising
pulse output means for generating stimulation pulses and applying
said pulses to the first terminal of said lead, whereby said
stimulation pulses are delivered to said first electrode of said
lead,
a sensing amplifier connected to said second terminal of said lead,
said sensing amplifier generating an output signal in response to
sensed electrical signals applied thereto through said second
terminal, said electrical signals being generated by spontaneous
and stimulated cardiac activity, and
means for determining capture, said means for determining capture
including means for monitoring the output signal of said sensing
amplifier, said monitoring means including means for measuring
successive time intervals between application of a stimulation
pulse through said first electrode and the generation of the output
signal by said sensing amplifier, said capture means providing an
indication of capture if said measured time interval remains
constant and corresponds approximately to a predetermined time
interval.
8. A cardiac pacing system comprising:
lead means for providing electrical contact with a desired cardiac
location, said lead means having first and second terminals at the
lead proximal end, a first electrode at the lead distal end being
electrically connected to said first terminal, a second electrode
near the lead distla end being spaced apart and electrically
insulated from said first electrode and being electrically
connected to said second terminal; and
pacemaker means comprising
first connector means for electrically engaging said first
terminal,
second connector means for electrically engaging said second
terminal,
pulse generating means connected to said first connector means for
providing a stimulation pulse,
sensing EGM amplifier means connected to said second connector
means for sensing electrical signals representative of cardiac
action and for generating an output signal in response thereto,
capture-determining means for determining if the stimulation pulse
generated by said pulse generation means has produced capture, said
capture-determining means including means for measuring the time
interval between application of a stimulation pulse by said pulse
generation means and the occurrence of the output signal from said
sensing EGM amplifier means, capture being indicated if said
measured time interval remains constant and corresponds
approximately to a predetermined time interval, and
adjustment means coupled to said pulse generating means for
increasing the energy of subsequent stimulation pulses in the event
said capture-determining means indicates capture has not
occured.
9. A method of determining capture of stimulation pulses comprising
the steps of:
(a) generating a stimulation pulse and applying it to a prescribed
location to be stimulated;
(b) sensing electrical activity at a location spaced apart from the
stimulating location of step (a);
(c) measuring the time delay between the application of the
stimulation pulse of step (a) and the sensing of electrical
activity of step (b); and
(d) adjusting the magnitude of the stimulation pulses until the
time delay measured in step (c) for successive measurements is
substantially the same and within a prescribed tolerance of an
expected propagation time delay, denoting captured stimulation
pulses.
10. The method of determining capture of claim 9 wherein step (c)
of adjusting the magnitude of the stimulation pulses comprises
adjusting the stimulation pulse width.
11. The method determining capture of claim 9 wherein step (c) of
adjusting the magnitude of the stimulation pulses comprises
adjusting the stimulation pulse amplitude.
12. The method of determining capture of claim 9 wherein step (c)
of adjusting the magnitude of the stimulation pulses comprises
decreasing the stimulation pulse magnitude until the time delay
measured in step (b) for successive measurements is not
substantially the same, indicating lack of capture, and
increasing the stimulation pulse magnitude in prescribed increments
until the time delay measured in step (b) for successive
measurements is substantially the same and within said prescribed
tolerance of an expected propagation time delay, indicating
captured stimulation pulses.
Description
TECHNICAL FIELD
The invention relates to implantable heart pacemakers, and more
specifically to heart pacemakers having a capability to stimulate
the atrium and sense the atrial response.
BACKGROUND ART
"Capture" is defined as a cardiac response to a pacemaker
stimulation pulse. When a pacemaker stimulation pulse stimulates
either the heart atrium or the heart ventricle during an
appropriate portion of a cardiac cycle, it is desirable to have the
heart respond to the stimulus provided Every patient has a
threshold which is generally defined as a minimum amount of
stimulation energy required to effect capture. It is usually
desired to achieve capture at the lowest possible energy setting
yet provide enough of a safety margin so that should the patient's
threshold increase, the output of an implanted pacemaker would be
sufficient to maintain capture.
Capture is usually assessed by means of an electrocardiogram (ECG)
measured through ECG electrodes placed on the patient's limbs
and/or chest. When a patient is connected to a typical ECG monitor
and the pacemaker is providing stimulation pulses, the physician
monitors the output to assess whether each pacing pulse, which is
seen as a spike, is followed by a cardiac response. Ventricular
capture is relatively easy to assess in that each ventricular
stimulation produces a very large R-wave. Determination of atrial
capture in response to an atrial stimulation pulse is a more
difficult task. Atrial capture in response to stimulation pulses
has been viewed on an electrocardiogram as P-waves following each
atrial stimulation by a constant time interval. One prior art
embodiment utilized dual sensing electrodes and suggests the heart
action is 15 to 20 milliseconds after the stimulus. (See Goldreyer,
U.S. Pat. No. 4,365,639.) However, the time delay varies
considerably depending on the patient, administered drugs,
electrolyte balance, proximity of sensing electrode to stimulating
electrode and other factors. Further, it is almost impossible to
guarantee that a P-wave will be of a sufficient amplitude to be
seen on a standard ECG scan. In order to verify atrial capture in
patients with intact cardiac conduction, the physician must pace
atrially and observe ventricular response to the paced atrial rate.
However in patients with heart block, the physician may not be able
to determine atrial capture because of the lack of conduction from
the atrium to the ventricle, thus preventing the ventricle from
responding to atrial stimulation pulses. In such cases the
physician may have to rely on fluoroscope evaluation of cardiac
wall motion in response to the atrial stimulation to ascertain
atrial or P-wave capture.
Another method for determining atrial capture is to transmit the
signal appearing on the atrial stimulation electrode to an external
viewing device. Some of the newer pacemakers have the capability to
transmit electrogram (EGM) signals appearing at either the atrial
electrode or the ventricular electrode in real time to an external
monitoring device for real-time evaluation by a physician. (See,
for example, U.S. Pat. No. 4,232,679 to Schulman.) However, due to
the large magnitude of a stimulation pulse with respect to the
P-wave signal, and the closeness in time between the stimulation
pulse and the occurrence of the P-wave, the atrial sensing
amplifiers of conventional pacemakers saturate in the presence of a
stimulation pulse and mask the P-wave. Thus as a practical matter,
utilization of EGM signals appearing at the stimulation electrode
is not effective for determining if P-wave capture has occurred.
One feature of the present invention solves this problem by
providing an apparatus for determining P-wave capture through use
of a conventional implantable bipolar atrial electrode without
having to utilize the stimulation electrode for P-wave
detection.
DISCLOSURE OF INVENTION
The invention provides a system useful for determining P-wave
capture in response to pacemaker supplied atrial electrical
stimuli. The system includes a first lead means having at its
distal end a first exposed electrically conductive electrode and a
second exposed electrically conductive electrode spaced apart from
the first electrode a distance no greater than that required for
the first and second electrodes to be operably located within a
user's heart atrium. The first electrode is electrically connected
to a first terminal means near the proximal end of the lead means,
and the second electrode which is electrically isolated from the
first electrode is electrically connected to a second terminal
means also near the proximal end of the lead means. The invention
further includes a pacemaker means which includes a first pulse
output means for generating atrial stimulation pulses, a first
connector means electrically connected to the first pulse output
means and adapted to engagingly receive the first terminal means, a
P-wave sensing EGM amplifier means for sensing P-waves generated by
spontaneous and stimulated atrial action, a second connector means
electrically connected to the input of the P-wave sensing EGM
amplifier means and adapted to engagingly receive the second
terminal means, and a means for monitoring the output of the P-wave
sensing EGM amplifier means in the presence of atrial stimulation
pulses generated by the pulse output means for the purpose of
determining if P-wave capture due to the atrial stimulation pulses
has occurred.
In a specific embodiment of the invention, the first lead means is
a conventional bipolar (two wires) atrial pacing lead having a
distal electrode at its tip, and a ring electrode spaced apart from
the tip, each electrode being electrically isolated from the other
and connected to respective terminals at the proximal end of the
lead. The ring electrode is connected to the input of a P-wave
sensing EGM amplifier having bandpass characteristics such that the
signals which normally appear on an atrial EGM in the presence of
atrial stimulation pulses can be detected. The distal electrode at
the tip is connected to a P-wave sense/pace amplifier and to an
atrial pulse output circuit, the P-wave sense/pace amplifier being
chosen to have a bandpass characteristic that allows positive
detection of P-waves in the absence of stimulation pulses while
discriminating against other non-P-wave signals present in the
atrium. A unipolar (single wire) ventricular lead is also provided,
this lead being connected to an R-wave sense/pace amplifier within
the pacemaker as well as to a ventricular pulse output circuit. The
outputs of the P-wave sensing amplifier, P-wave sense/pace
amplifier, and R-wave sense/pace amplifier are provided to a switch
controllable through a telemetry subsystem for selecting which of
the amplifier outputs will be telemetered in real time to a
remotely located monitor for physician analysis. Other embodiments
of the invention provide for automatic setting of stimulation pulse
amplitudes in response to a determination of P-wave capture
thresholds.
It is obvious that the stimulating electrode need not be at the tip
or that the second electrode be a ring. Basically, they must simply
be adapted to be spaced apart in the heart. The structures
mentioned, however, are preferred.
In operation, the ring electrode being spaced apart from the
stimulation electrode does not receive the high energy output
present at the stimulation electrode during atrial stimulation.
This is because of attenuation within the heart itself and because
the ring electrode is not connected to the output circuit. Thus,
the bandpass characteristics of the P-wave sensing amplifier are
chosen so that a P-wave as well as other electrical signals within
the atrium can be detected immediately after an atrial stimulation
pulse. By telemetering the output of the P-wave sensing amplifier
to a monitor for analysis by a physician, the physician can
determine if P-wave capture has occurred by looking at the time
differential between the atrial stimulation pulse and the
occurrence of a P-wave. If this time differential remains constant,
and if it is of a proper duration as determined by the spaced-apart
distance between the first and second electrodes, the physician may
conclude that P-waves are being generated as a result of the atrial
stimulation pulses rather than spontaneous, or native, atrial
activity. If on the other hand, the time differential between the
occurrence of the stimulation pulse and the P-wave varies from
cycle to cycle, then the physician can assume that capture has not
occurred and that the P-waves appearing on the trace are being
generated by spontaneous atrial activity. In this event, the
physician can increase the amplitude or the pulse width of the
stimulation pulses until such time as the stimulation pulse/P-wave
occurrence time interval is constant between successive cycles,
thereby indicating that the stimulation pulses are causing P-wave
generation. It is possible, although unlikely, that spontaneous
P-wave generation is present at a frequency equal to that of the
atrial stimulation pulses. However, because the distance from the
distal electrode in the atrium to the ring electrode is known, and
the distance from the ring electrode to the heart sinus node can be
estimated, and the propagational characteristics of atrial
stimulation events within the heart are known, the physician can
also measure the time differential between occurrence of the atrial
stimulation pulse and the P-wave and determine if this time
differential corresponds to the distance between the distal
electrode and the ring electrode or to the distance between the
sinus node and the ring electrode This provides one manner in which
the physician can determine if P-wave capture has occurred due to
atrial stimulation pulses.
In some implantations, the ring electrode may be at or near the
sinus node. Atrial capture can still be determined in accordance
with the invention.
In a further embodiment of the invention, a pacemaker is disclosed
which has the capability to automatically adjust the amplitude of
the atrial stimulation pulses until a sufficient amplitude is
maintained to effect atrial P-wave capture.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of a pacemaker according to the present
invention;
FIG. 2 is a waveform diagram illustrating the problem of P-wave
detection utilizing the atrial stimulation electrode as a P-wave
sensing electrode;
FIG. 3 is a schematic representation of a user's heart showing
locations of the atrial and ventricle electrodes;
FIGS. 4A and 4B are waveform diagrams of intercardiac electrogram
(EGM) signals, as viewed by a physician on programmer 20 of FIG. 1
FIG. 4A illustrating an EGM signal showing P-wave capture and FIG.
4B illustrating an EGM signal in the absence of P-wave capture;
FIG. 5 is a block diagram of a P-wave amplifier shown in FIG.
1;
FIG. 6 shows frequency response curves for the P-wave sense/pace
amplifier and the P-wave sensing amplifier;
FIG. 7 is a partially cut-away atrial electrode utilized in the
embodiment of FIG. 1;
FIG. 8 is a block diagram of a further embodiment of the invention;
and
FIG. 9 illustrates a pulse voltage/pulse width curve showing P-wave
capture as a function of pulse amplitude and pulse width.
BEST MODE FOR CARRYING OUT THE INVENTION
Detailed illustrative embodiments of the invention disclosed herein
exemplify the invention and are currently considered to be the
preferred embodiments for such purposes. They are provided by way
of illustration and not limitation of the invention. Various
modifications thereof will occur to those skilled in the art, and
such modifications are within the scope of the claims which define
the present invention.
In the preferred embodiment, a heart pacing system is disclosed in
which a conventional bipolar atrial lead is utilized in conjunction
with an implantable pacemaker having one P-wave sense/pace
amplifier attached to an atrial stimulation electrode at the lead
distal end, or tip, and another P-wave amplifier attached to a
spaced-apart ring electrode near the distal end of the lead. The
P-wave sense/pace amplifier electrically connected to the
stimulation electrode has bandpass characteristics which allow a
P-wave to be detected in the absence of a stimulation pulse and
discriminate against non-P-wave signals present in the atrium. The
output of the P-wave sense/pace amplifier is used to inhibit atrial
stimulation pulse generation in the presence of a
naturally-occurring P-wave. The other P-wave amplifier attached to
the ring electrode has bandpass characteristics chosen to detect
both naturally occurring P-waves and P-waves in response to
stimulation pulses for the purpose of determining if P-wave capture
has occurred. Whenever the time periods between stimulating pulses
and the next-occuring P-waves are substantially constant, the
physician can be generally assured that P-wave capture has
occurred. The amplitude of the stimulation pulse can then be
lowered to such a value that P-wave capture can be maintained with
the least possible power drain from the pacemaker battery. The
pacemaker disclosed is typically of the DDD type in which pacing
and sensing in both the atrium and ventricular chambers of the
heart are possible. Accordingly, a second ventricle lead and an
associated R-wave sense/pace amplifier within the pacemaker are
also provided. A switch responsive to signals received through the
pacemaker telemetry system is also provided so that the output of
either P-wave amplifier, or the R-wave amplifier can be transmitted
in real time to an external monitor for analysis by a physician. It
should be recognized however that the invention is not limited to a
DDD type pacemaker and other pacemaker types could be utilized such
as AAI and AAT pacemakers for example.
Referring now to FIG. 1, a block diagram of an implanted pacemaker
16 according to the invention is shown, the pacemaker 16 being
connected to a user's heart 18 and electromagnetically in, contact
with an external programmer 20, which programmer 20 includes a
telemetry transmitter and receiver and monitor external to the
user's skin 21. A conventional bipolar atrial lead 22 is provided
having a first or tip electrode 24 at its distal end and a second
electrode 26 spaced apart from the tip electrode 24 and in the
configuration of a typical bipolar lead ring electrode. It may be
understood that a second ring electrode and an associated amplifier
may be used for greater signal strength in sensing the electrical
activity in the atrium. The tip electrode 24 is located in contact
with atrial tissue of the heart atrium 28. A unipolar ventricle
lead 30 is located in the heart ventricle 32 and is attached to the
pacemaker 16 through a ventricular connector 34. The atrial lead 22
is connected to the pacemaker 16 through an atrial connector 36.
The pacemaker 16 includes a telemetry subsystem 40 for transmitting
data and parameter values to the external telemetry transmitter and
receiver of the external programmer 20, and for receiving data
instructions and the like from the external programmer 20. The
pacemaker 16 also includes pulse generator logic circuitry 42
which, in turn, controls pulse output circuits 44 for providing
both atrial and ventricle stimulation pulses. The atrial output of
the pulse output circuits 44 is connected through the atrial
connector 36 to the atrial tip electrode 24 for stimulation of the
atrium; the ventricle output of the pulse output circuits 44 is
connected through the ventricle connector 34 to a ventricle tip
electrode 46 for stimulation of the ventricle. A P-wave sense/pace
amplifier 48 having bandpass characteristics as explained below is
also connected through the atrial connector 36 to the atrial tip
electrode 24 for receiving electrical signals present at the
electrode 24. The output of the P-wave sense/pace amplifier 48 is
also connected to the pulse generator logic circuitry 42 and to an
ECG switch 50, the purpose of which will be explained below. The
implanted pacemaker, in operating as a "demand" type pacer, would
not provide stimulation to the atrium when amplifier 48 provided
its output indicating an intrinsic P-wave. A second amplifier, a
P-wave sensing EGM amplifier 54 having bandpass characteristics as
explained below has its input connected through the atrial
connector 36 to the second atrial electrode 26. The output of the
P-wave sensing amplifier is also connected to the switch 50. An
R-wave sense/pace amplifier 56 is also provided, its input being
connected to the pulse output circuits 44 and the ventricle tip
electrode 46 through the ventricle connector 34. The output of the
R-wave sense/pace amplifier 56 is connected to the pulse generator
logic circuitry 42 for inhibiting a ventricle stimulation pulse in
the presense of spontaneous ventricular activity, and to the switch
50. Amplifier 56 has a sufficiently broad band-pass to pass
electrical signals of substantially all native (intrinsic)
ventricular activity. The output of the switch 50 is connected via
a line 58 to the telemetry subsystem 40 for real time transmission
of the output of either the P-wave sense/pace amplifier 48, the
P-wave sensing amplifier 54 or the R-wave sense/pace amplifier 56.
The specific amplifier output to be transmitted is selected by the
physician via instructions transmitted by the external telemetry
transmitter and receiver 20 and received by the implanted telemetry
subsystem 40. These instructions are decoded by a decoder and
encoder 60. The output of the decoder and encoder 60 is utilized to
establish which amplifier output 48, 54 or 56 is to be connected to
the telemetry system 40 for transmission to the external telemetry
transmitter and receiver 20. Although the switch 50 is shown as a
switch, it should be readily apparent that any kind of selectable
connecting means could be employed to provide continuity between
one of the amplifiers 48, 54 and 56 and the line 58. Further, two
or more of the amplifier outputs could be transmitted
simultaneously if proper provisions were made within the telemetry
subsystem 20. In addition, a memory 62 is provided which receives
parameter information from the decoder and encoder 60, this
parameter information being utilized to control the pulse generator
logic circuitry 42. The tip electrode 24 for stimulating the atrium
and the tip electrode 46 for stimulating the ventricle are all
utilized in a unipolar configuration with the return path being
provided through a conductive portion of the pulse generator case
64 which is connected to the pulse output circuits 44. A battery 66
is also incorporated for providing power to the implanted pacemaker
16. It should also be recognized that although an implanted
pacemaker is shown for illustrative purposes, the invention is in
no way limited to an implanted pacemaker. An external pacemaker
could also be provided in accordance with the teachings of the
invention. Further, although a unipolar ventricular lead was chosen
for illustrative purposes, a bipolar ventricular lead could also
have been utilized provided appropriate connectors were available
on the pacemaker. Similarly, a multi-conductor atrial lead could be
provided with two of the conductors providing a bipolar atrial lead
and the third conductor being connected to the P-wave sensing ECG
amplifier 54 shown in FIG. 1.
Operation of the implanted pacemaker 16 shown in FIG. 1 can be best
understood by reference to FIGS. 2, 3, 4A and 4B. As previously
explained, one of the problems associated with atrial pacing is
determining whether P-wave capture has been effected by atrial
stimulation pulses. In prior art systems, the sensing circuit
corresponding to the P-wave sense/pace amplifier 48 in FIG. 1
sensed signals present at the electrode at the lead distal end
which corresponds to the tip electrode 24 in FIG. 1. Referring now
to FIG. 2, the voltage present at the output of the P-wave
sense/pace amplifier 48 in the presence of an atrial stimulation
pulse corresponds in general to the waveform shown at 70. Thus, the
output of the P-wave sense/pace amplifier 48 is saturated during
the period "S" shown in FIG. 2. Because the P-wave voltage is small
with respect to the saturation voltage caused by the stimulation
pulse as shown by the dotted line 72, it is difficult, if not
impossible, to pick out the time at which the P-wave occurred
relative to the stimulation pulses which occur at the times
indicated by the arrows 74. Because of this difficulty in
determining when the P-wave 72 actually occurred relative to
stimulation pulse occurrence as shown at 74, it is difficult for
the physician to determine if the stimulation pulse has effected
P-wave capture.
Referring now to FIG. 3, a representation of the heart is shown,
showing a pacemaker 76 according to the invention, having a
conventional atrial lead 78 and, also having a stimulation and
sensing electrode 80 at its distal end and a second or P-wave
sensing electrode 82 spaced apart from the stimulation electrode
80. By way of example only, the atrial lead 78 is configured in the
form of a J at its distal end so that the stimulation electrode 80
can be located within the atrial appendage (not shown). The heart
sinus node 84 is also shown, as well as a ventricle lead 86 having
its stimulation electrode 88 located in the ventricular apex. It
can be appreciated that the further the sensing electrode 82 is
spaced-apart from the stimulation electrode 80, the less the
stimulation pulses will interfere with P-wave sensing by the
sensing electrode 82. This is because of an attenuation of the
electrical stimulation signal due to intervening blood and heart
tissue. However, it should be apparent that the sensing electrode
82 cannot be so far removed from the stimulation electrode 80 that
it would no longer be within the heart atrium. All electrodes 80,
82 and 88 use the case of the pacemaker 76 as a return electrode,
the case being positive with respect to a negative going pulse
present at both stimulation electrodes 80 and 88. Another advantage
of utilizing the spaced-apart sensing electrode 82 for P-wave
detection is that the P-wave electrical characteristics as picked
up by the sensing electrode 82 differ because of the direction of
propagation as shown by the arrows 90 and 92, arrow 90 showing the
propagation direction from the stimulation electrode 80 and arrow
92 being the propagation direction from the sinus node 84. This
allows the physician to determine if the P-wave occurred as a
result of spontaneous atrial activity or stimulated atrial
activity. Further, because of the different distances between the
sensing electrode 82, the sinus node 84 and the stimulation
electrode 80, it can be appreciated that even if the sinus node 84
is operating in synchronism with the stimulation pulses, the known
propagation time between a stimulation pulse and P-wave generation
could be used to determine if P-wave generation were due to
spontaneous or stimulated atrial activity. Further, it can be
appreciated that although a typical bipolar atrial electrode 78 is
utilized, all three electrodes 80, 82 and 88 operate in a unipolar
manner in that they all use the pacemaker 76 case as a common
return electrode.
Detection of atrial capture can be further understood in reference
to FIGS. 4A and 4B. Referring to FIG. 4A, atrial stimulation pulses
100 and ventricular stimulation pulses 102 can be seen. Further,
P-waves 104 and R-waves 106 can also be seen. The time differential
D between atrial stimulation and P-wave occurrence in successive
cycles can be seen to be constant. Thus, the physician can assume
that P-wave capture as a result of the atrial stimulation pulses
has occurred provided that the distance D corresponds approximately
to the propagation delay due to the distance between the
stimulation electrode 80 and the sensing electrode 82 as explained
in conjunction with FIG. 3. Referring now to FIG. 4B, the time
differentials D' and D" between the atrial stimulation pulses and
P-wave occurrences can be seen to be different. Thus, the physician
can conclude that P-wave capture by the atrial stimulation signals
has not occurred but that the P-waves are spontaneous or "native"
or "intrinsic" in origin. Under normal circumstances with respect
to FIG. 4B, the physician would assume that the magnitude of the
stimulation pulses is below the stimulation threshold of the
particular patient's atrium and would accordingly increase their
magnitude until P-wave capture occurred, that is, until an EGM
signal similar to that shown in FIG. 4A is observed on the monitor
of the external programmer 20. Again, in prior art systems, it
would be impossible to observe the presence of P-waves utilizing
the stimulation electrode 80 as the sensing electrode due to
saturation of the P-wave sense/pace amplifier caused by the atrial
stimulation pulse.
FIG. 5 shows a simplified block diagram of a typical P-wave or
R-wave wave amplifier 109 such as those shown in FIG. 1 as blocks
48, 54 or 56. The amplifier 109 includes an amplification portion
110 and an input filter 112. The difference between the P-wave
sense/pace amplifier 48 and the P-wave sensing amplifier 54 is in
the bandpass characteristics of the amplification portion 110 and
filter 112 combination. The amplitude and bandpass characteristics
of the P-wave sense/pace amplifier 48 are chosen to provide to the
pulse generator logic circuitry 42 a positive indication of P-wave
occurrence in the absence of an atrial stimulation pulse, while at
the same time rejecting non-P-wave signals such as far-field R-wave
signals and muscle electrical noise. This is to allow the pulse
generator logic circuitry 42 to determine if the atrium is
operating spontaneously or whether an atrial stimulation pulse is
required. The purpose of the P-wave sensing amplifier 54 is to
provide an electrogram of all, or most all, atrial electrical
action including an indication of P-wave occurrence in the presence
of an atrial stimulation pulse. Thus, the precise characteristics
of the P-wave and its location with respect to an atrial
stimulation pulse must be analyzed to determine if the P-wave is
occurring spontaneously or is occurring as a result of an atrial
stimulation pulse. In order to meet these different requirements,
the amplification portion 110 filter 112 combination of the P-wave
sense/pace amplifier 48 as shown in FIG. 1 can be chosen to have a
center frequency at 60 Hz and 3 db points at approximately 10 Hz
and 100 Hz. The purpose of this U-shaped frequency response is to
maximize detection of the P-wave which has a large frequency
component near 60 Hz and to reject other signals such as some of
that from the heart R-wave which by the time it reaches the atrium
has lower frequency components and muscle electrical noise which
has higher frequency components. Thus, the bandpass characteristics
of the P-wave sense/pace amplifier must be chosen to attenuate all
electrical signals within the atrium other than the one frequency
that most characterizes the P-wave. Of course the bandpass
characteristics described above are only representative of one
embodiment, and other U-shaped response characteristics could be
chosen. For example, the peak of the response curve could be chosen
to lie between 40 Hz and 80 Hz and the 3 db points could lie
between 0.1 Hz and 500 Hz. The teaching of the invention merely is
that the P-wave sense/pace amplifier be chosen to pass signals
characteristic of the P-wave while tending to reject signals that
are not characteristic of the P-wave. Thus peak detection circuitry
in the pulse generator logic circuitry 154 can be triggered by the
output of the P-wave pace/sense amplifier 44 without danger of a
false detection due to other electrical activity in the atrium.
The P-wave sensing EGM amplifier 54 is chosen to have a response
that is essentially flat between 31/2 Hz and 125 Hz. This is to
allow the physician to see all electrical atrial activity for a
complete understanding of the atrial electrical environment
including any T-wave ventricle signals and any far-field R-wave
signals that are present. However, the invention is in no way
limited to a P-wave sensing EGM amplifier having a flat response,
and a U-shaped frequency response such as that of the P-wave
sense/pace amplifier could also be utilized. However, use of such a
U-shaped frequency response would limit the EGM information
available to the physician without any compensating advantage.
The above can be further understood by referring to FIG. 6. Here
the P-wave sense/pace amplifier response 113 and the p-wave sensing
EGM amplifier response 114 can be seen. As can be seen, the
response 113 is chosen to pass the p-wave frequency and attenuate
the frequencies associated with other physiologic events as shown
at 115 and 116 in order to provide a relatively high amplitude
output corresponding only to R- and P-wave events. The response 114
is chosen to pass all frequencies in order to provide an accurate
overall EGM signal to the physician. As shown, response 114, of the
sensing amplifier includes T-wave ventricular frequencies and
far-field signal.
Referring now to FIG. 7, a lead 22' of the type shown in FIG. 1 as
22 is illustrated. Although a straight shank lead is shown for
illustrative purposes, it should be recognized that a typical
atrial-J lead could be utilized in the application shown in FIG. 1,
and thus a portion of the distal end of the lead could be J-shaped.
The lead 22' includes a tip electrode 24' which is connected
through a spirally-wound conductor 117 to a first terminal 118. A
ring electrode 26' is attached through a spirally-wound conductor
122 to a second terminal 120, this conductor 122 being electrically
isolated from the conductor 117 attached to the tip electrode 24'.
The terminals 118 and 120 are adapted to connect to appropriate
connectors in the pacemaker. Although the connector or terminal
arrangement generally shown at 124 is a typical in-line type of
connector, other connector arrangements could be utilized such as
having each terminal coming out of the proximal end of the lead to
form a Y-shaped connector. The ring electrode 26' is spaced apart
from the tip electrode 24' a distance such that when the tip 24' is
located in the atrial appendage, the ring electrode 26' will also
be located within the atrium. As previously explained, FIG. 7
merely illustrates a typical bipolar atrial lead which is utilized
in the FIG. 1 embodiment while having its tip electrode and ring
electrode operate in a unipolar fashion. Thus, an implantable
pacemaker configured according to that shown in FIG. 1 can be
utilized with conventional bipolar atrial leads without requiring a
special purpose lead to be utilized.
A further embodiment of the invention is shown in FIG. 8. This
embodiment incorporates a means for automatically controlling the
characteristics of the output atrial stimulation pulse in order to
select an amplitude and/or width that would maintain P-wave capture
without utilizing an unnecessarily large amplitude and/or width
that would result in high battery current drains leading to
premature battery depletion. Referring now to FIG. 8, an
implantable pulse generator 130 is shown, the pulse generator 130
being connected through connectors 36' and 34' to the user's heart
132 through a conventional bipolar atrial-J lead 22' and a unipolar
ventricle lead 30' in a manner identical to that as explained in
conjunction with FIG. 1. Other components similar to those shown in
FIG. 1 include the pulse output circuits 44', the switch 50', the
P-wave sense/pace amplifier 48', the P-wave sensing amplifier 54',
the R-wave sense/pace amplifier 56', the pulse generator case 64'
and the ventricle lead connector 34'. The switch 50' operates in
the same manner as the switch 50 shown in FIG. 1 in that the
outputs of the P-wave sense/pace amplifier 48', the P-wave sensing
amplifier 54' and R-wave sense/pace amplifier 56', are provided to
the switch 50', which in turn is controlled by signals from other
portions of the pacemaker to select which of the specific amplifier
48', 54' or 56' output signals will be transmitted in real time via
a telemetry subsystem 136 to an external telemetry transmitter and
receiver 138. The information from each of the amplifiers and, if
desired, from the memory or other portion of the pacemaker may be
displayed such as an electrocardiograph (ECG) 156. The output of
the P-wave sensing amplifier 54', which is connected to the ring
electrode 26' shown in FIG. 8 for the purpose of detecting P-waves
in the presence of atrial stimulation pulses, is connected to a
P-wave detector 140 for providing an output pulse synchronized with
the occurrence of a sensed P-wave. The P-wave detector 140 includes
conventional circuitry which produces a pulse output whenever the
P-wave signal exceeds a predetermined level for a predetermined
time. The P-wave detector also has an input bandpass characteristic
similar to that of the P-wave sense/pace amplifier shown at 113 in
FIG. 6 in order to attenuate other electrical signals in the atrium
as explained above. The P-wave detector 140 output is connected to
a reset terminal of a monostable flip-flop 142. A counter 144 is
provided having a start terminal connected to the Q output of
flip-flop 142 and a stop terminal connected to the Q output of
flip-flop 142. The set input to flip-flop 142 and the reset input
of the counter 144 are connected to the atrial stimulation output
of the pulse output circuits 44'. The output of the counter 144 is
connected to a comparator 146, the output of which is connected to
a pulse amplitude, width controller 148. The comparator 146 and the
pulse amplitude width controller 148 are further controlled by
signals from a memory 150. Further, a decoder and encoder 152 is
provided, the decoder and encoder 152 being connected to the
telemetry subsystem 136 and the memory 150, and providing an output
to the switch 50' for controlling which of the P-wave or R-wave
amplifier outputs are to be transmitted in real time to the
external transmitter and receiver 138 by the telemetry subsystem
136. The comparator 146 can be of any standard type well-known in
the art, either digital or analog, that compares the elapsed time
between the start and stop inputs to the counter 144 for
determining time coincidence. If the elapsed times between the
start and stop pulse inputs to the counter 144 are equal for
successive stimulation pulse/P-wave intervals, then the pulse
amplitude, width controller 148 is chosen to reduce the output
voltage of the atrial stimulation pulse through the pulse generator
logic circuitry 154 until such time as the output of the comparator
146 indicates that the elapsed time intervals between start and
stop inputs to the counter 144 are, not equal. At that point, the
pulse amplitude, width controller 148 increases the amplitude of
the atrial stimulation pulses until the counts provided by the
counter 144 are equal. As can be readily appreciated, this
comparison can be effected digitally or can be effected by numerous
analog circuits. A further refinement in the comparator 146, which
again can be readily implemented by one familiar with electronic
circuits, would be to reject start and stop elapsed times that
correspond to the propagation time between the heart sinus node and
the ring electrode 26' as compared with the propagation time
between the distal electrode 24' and the ring electrode 26' as
previously explained. Thus, if the atrium were functioning in
synchronism with the atrial stimulation pulses, but P-wave capture
had not been effected, this additional feature would prevent the
system from operating as though P-wave capture had been achieved.
Thus, one can readily appreciate that an implantable pacemaker has
been disclosed in which through appropriate signals from the
external transmitter and receiver 138, through the telemetry
subsystem 136, the decoder and encoder 152 and the memory 150, the
amplitude of the atrial stimulation pulses can be automatically
lowered until P-wave capture is lost, and then slowly increased
until P-wave capture is achieved, thus resulting in the most
efficient use of battery power and maximization of the life of the
implanted pacemaker. Further, the pacemaker through its memory 150
could be programmed to automatically effect this resetting of
atrial stimulation pulse amplitude at predetermined time intervals
such as every month without requiring the presence of an attending
physician. Although not shown, it can be readily appreciated that
such a system could also be utilized to set the output of the
ventricular stimulation pulses. However, in the ventricle, the
determination of R-wave capture is a less critical problem because
of the magnitude of the R-wave with respect to the magnitude of the
ventricle stimulation pulse.
In a still further embodiment of the invention, the memory 152 can
be programmed using conventional techniques to set the atrial
stimulation plus amplitude and pulse width optimally for
conservation of battery power. Referring to FIGS. 8 and 9, the
pulse generator logic 154 can be readily programmed through the
memory 150 to set various pulse widths and determine the
appropriate voltage thresholds for P-wave capture as explained in
the discussion associated with FIG. 8. If, for example, threshold
voltages for pulse widths in 0.1 millisecond increments between 0.1
milliseconds and 1.0 millisecond were taken, a P-wave threshold
voltage versus pulse width curve as shown at 170 in FIG. 9 can be
derived. For each pulse width increment reading, the current drain
for the threshold stimulation pulse is measured and the power
consumed computed by the pulse generation logic circuitry. The
current measurement can be accomplished by several conventional
methods all of which are well known in the art. From the known
current utilized, and the voltage of the stimulation pulse, the
power can be readily computed The power for each threshold measured
is stored in the memory 150 and the pulse amplitude/pulse width
combination providing the lowest power drain chosen Upon
determining the pulse amplitude/pulse width combination that
provides the lowest power drain, which may for example be 0.3
milliseconds at 2 volts as shown in FIG. 9, a delta pulse width DW
and a delta pulse amplitude DA is programmed to be added to provide
a margin of safety, or margin of operability. This margin of safety
when applied to all the measurement points provides an "actual
setting" curve shown at 172 in FIG. 9. The memory can be programmed
to initiate this sequence at appropriate intervals, for example at
one month intervals, to periodically ensure that the optimum pulse
amplitude/pulse width combination is maintained.
Thus, a system has been disclosed whereby either the physician or
the implanted pacemaker itself can determine if P-wave capture by
atrial stimulation pulses has been effected, a capability that
heretofore has been unachievable with conventional pacing
systems.
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